This invention relates to air pollution analyzers and the like which measure constituents in a sample gas stream. More particularly, the invention relates to a combination cryotrap and adsorbant traps used for extracting constituents from the sample gas stream.
Non-Methane Organic Compounds are defined in and by 40 CFR Appendix A Method 25. The historical data for this methodology therefore must be comparable to future data to ensure all of the data is legally defensible. Thus, any direct measurement systems must be able to be favorably compared with earlier non-directly measured analyses. The only proven method for this comparison is with a physical trapping procedure using cryogenic processes.
In a typical air pollution application, the air sample is passed through a cryotrap which is maintained at the temperature of dry ice. Heavier molecular weight constituents in the gas sample are condensed by the low temperature and solidify on the cold surfaces of the trap while lighter molecular weight constituents are separated or absorbed by the columns. After the desired amount of sample gas has passed through the trap, the gas flow path is changed so as to pass a carrier gas through the trap in place of the sample gas. At this time, the traps are heated. The frozen constituents vaporize, the absorbed constituents are desorbed, and both are carried away with the carrier gas to an analyzer or other instrument. The now clean traps are ready for another sampling and flushing cycle.
In the cryotraps commonly used, liquid nitrogen, liquid carbon dioxide, or solid dry ice are chosen for the freezing step. However, the coolant is lost to the atmosphere and a new quantity must be utilized for each operation. For instruments which operate continuously, the cost of these coolants is very high.
It is an object of the present invention to provide a new and improved cryotrap which uses a closed cycle cooler for the freezing, with no coolant being discharged after each sampling cycle. A further object of the invention is to provide such a cryotrap with a new and improved construction for obtaining increased efficiency in cooling and heating while enabling smaller construction and shorter operating cycles.
Earlier mechanical coolers are bulky, heavy, noisy, and have high mechanical vibration. They have relatively short life cycles or had long cycle times. The present invention is to use a Stirling closed cycle linear cooler which is relatively lightweight and compact. Features of the Stirling cooler include low conducted and radiated emissions, low mechanical vibration, acoustically quiet operation, and improved lifetime and reliability. The energy transfer is much greater and allows for a faster cooling cycle.
Other objects, advantages, features and results will more fully appear in the course of the following description.